An audio signal performs coding and decoding generally in a frequency domain, for example, advanced audio coding (AAC). For example, AAC codec performs the modified discrete cosine transform (MDCT) for transformation to the frequency domain, and performs frequency spectrum quantization using a signal masking level in an aspect of psychoacoustic technology. Lossless coding is applied to further compress the quantization result. The AAC uses Huffman coding for the lossless coding. The bit-sliced arithmetic coding (BSAC) codec which applies arithmetic coding, may also be used instead of the Huffman coding for the lossless coding.
A speech signal is coded and decoded generally in a time domain. Mostly, a speech codec performing coding in the time domain is a code excitation linear prediction (CELP) type. The CELP refers to a speech coding technology. Currently, G.729, AMR-NB, AMR-WB, iLBC EVRC, and the like are generally used as the CELP-based speech coding apparatus. The coding method is developed under the presumption that a speech signal can be obtained through linear prediction (LP). When coding speech signals, an LP coefficient and an excitation signal are necessary. Usually, the LP coefficient may be coded using a line spectrum pair (LSP) while the excitation signal may be coded using several codebooks. The CELP-based coding method includes algebraic CELP (ACLEP), conjugate structure (CS)-CELP, and the like.
In an aspect of limit of transmission rate and psychoacoustics, a low frequency band and a high frequency band have a difference in sensitivity. The low frequency band is sensitive to fine structures on a speech/sound frequency. The high frequency band is less sensitive to the fine structures than the low frequency band. Based on this theory, the low frequency band is allocated with a great number of bits to code the fine structures in detail whereas the high frequency band is allocated with fewer bits than the low frequency band and performs coding. According to such a technology, such as spectral band replication (SBR), in the low frequency band, fine structures are coded in detail using a codec such as the AAC and, in the high frequency band, the fine structures are expressed by energy information and regulatory information. The SBR copies low frequency signals in a quadrature mirror filter (QMF) domain, thereby generating high frequency signals.
A bit reduction method is also applied to a stereo signal. More specifically, the stereo signal is converted to a mono signal, and a parameter expressing stereo information is extracted. Next, the stereo parameter and the mono signal compressed data are transmitted. Therefore, a decoding apparatus may decode the stereo signal using the transmitted parameter. For the compression of stereo information, a parametric stereo (PS) may be used. Also, a moving picture expert group surround (MPEGS) technology may be used to extract and transmit a parameter of a multichannel signal.
Objects of the lossless coding will be descried in further detail. The lossless coding may be performed, by regarding a quantization index of the quantized spectrum as one symbol. Also, coding may be performed by mapping the quantized spectrum index on a bit plane and collecting bits.
When context-based lossless coding is performed, information of a previous frame may be used. When the decoding apparatus does not have the information on the previous frame, a lossless decoding apparatus may perform wrong decoding or, even worse, the system may stop. For example, in digital audio broadcasting that applies an audio coding and decoding apparatus, a listener may turn on a radio and start listening at a random time. During the random access to an audio stream, the decoding apparatus needs information on the previous frame for precise decoding. However, reproduction is difficult due to lack of the previous frame information.